a. Field of the Invention
This invention relates generally to a distributed computing environment. More particularly, it relates to a method for use in a cluster of processors, wherein each processor in the cluster can access any disk in the cluster.
b. Related Art
The availability of powerful microprocessors has made clusters an attractive alternative to monolithic systems. Applications that can partition their computation among several nodes can take advantage of this architecture, which typically offers better price-performance than the monolithic systems. Such applications include large scientific computations, database and transaction processing systems, decision support systems, and so on.
A microprocessor cluster consists of a number of separate computing systems, coupled with an interprocessor communication mechanism, such as a network or communications switch. Each computing system has its own processor, memory, and I/O subsystem, and runs a separate instance of the operating system. For maximum benefit, however, it is desirable for an application to be able to abstract from the specific computing system, and treat all nodes in a cluster as equivalent. This ability is sometimes called a "single system image."
A useful aspect of single system image is the requirement that the same I/O device resources be available to all processors in the cluster equally. This allows processing tasks to be freely moved between processors. Furthermore, it facilitates the development of parallel applications that adopt a data sharing model for their computation.
Many different approaches can be taken to providing the same I/O resources to all processors, preferably in a highly available fashion. Data replication is the simplest, especially for read-only data, but it increases cost (resources not shared) and presents difficulties when the information changes over time.
An alternative is to have devices that can be physically attached to many processors. For example, twin-tailed (dual ported) disks are common. It is possible to build four-tailed disks, and even eight-tailed disks, but they become increasingly expensive and difficult to operate.
In both of the above cases, each processor has independent access to the resources, so no action is necessary to provide continuous access to the data in case of processor and/or adapter failure.
Distributed file systems, such as NFS, AFS and DFS, abstract away from the specific I/O device to the services it is intended for and provide those services to the processors in the cluster. This restricts the use of the device to those services, thus making it inappropriate for applications that are explicitly aware of the location of the data in the memory hierarchy. For example, a database system may rely on its own buffering and may want to arrange the data on disk in its own way, rather than rely on a file system to provide these services. In this case, direct access to the I/O device may be preferred by the application.
In terms of high availability, HA-NFS presents NFS clients with a highly available NFS server, but it relies heavily on the underlying network technology (IP address takeover) to provide critical functions that enable high availability.